The invention relates to a method for producing a pressure vessel from a metal liner. The invention furthermore relates to a method for producing a hydrogen pressure vessel, in particular for a motor vehicle.
Modern drive systems for motor vehicles may be based on the use of gaseous hydrogen as a source of energy. For this purpose, the hydrogen is installed and carried in the motor vehicle in a “hydrogen tank” at up to 700 times excess pressure. Hydrogen tanks of this kind are typically in the form of steel cylinders that have been used at up to 200 times excess pressure. To achieve a higher filling pressure, the wall of the hydrogen tank can additionally be composed of a fiber composite material containing glass and/or carbon fibers, for example.
To enable such gas tanks to be filled, they typically have a filler stub for feeding in and discharging the hydrogen, or for supplying a drive system with the hydrogen required.
DE 000010156377 A1, for example, discloses a gas pressure vessel which consists of a composite structure comprising a cylindrical metal container (liner) and a pre-produced jacketing tube, in particular a laminate tube. To produce the composite gas pressure vessel, a cylindrical metal container is introduced into the pre-produced jacketing tube and is deformed and expanded by means of gas pressure to such an extent that a nonpositive and permanent connection is formed between the metal container and the jacketing tube. The liner is composed of an aluminum alloy and the jacketing tube is composed of a carbon fiber laminate, for example.
One aim, particularly in the automotive industry, is that of reducing the weight of all the components and therefore also that of the onboard pressure tanks. To reduce the weight of gas pressure cylinders, composite gas cylinders (composite cylinders) are used, for example. Composite gas cylinders consist of a “liner”, which is wound over a significant part of its length with composite fibers composed of glass, carbon, aramid or wire by use of a special winding technique. For example, in the predominantly practiced wet-winding method, a fiber/resin laminate is applied to the liner in a controlled winding process and is then given its final usage properties in a downstream heat treatment process. Composite gas cylinders and the production thereof are described in DE 31 03 646 C2, DE 38 21 852 A1 and U.S. Pat. No. 3,843,010, for example. DE 10 2006 051 376 A1 discloses a further pressure vessel. This is prepared from a metallic material in a rolling process. To increase its strength or to obtain a thinner metallic wall in order to reduce weight, the metallic outer surface of the vessel was likewise wound with fiber-reinforced plastic.
Further improvements have been achieved by way of autofrettage. The term “autofrettage” is used to denote a method for increasing the service life of metal hollow bodies, especially fiber-reinforced metal hollow bodies, for use at high internal pressures. During this process, the metal hollow body is subjected to an internal pressure greater than the subsequent operating pressure, resulting in the metallic liner becoming plastic. After release of the pressure, internal compressive stresses arise in this region, preventing cracking in subsequent use and thus increasing fatigue strength right up to the endurance limit. An example of this kind is likewise found in the publication DE 102011007361 A1.
In the case of the wet-winding processes described above for jacketing the liner with a layer of fibers, the disadvantage is that there is an unwanted introduction of air into the resin system in the various process steps. The enclosed air bubbles lead to increased porosity and hence to lower fiber volume density. The results are, on the one hand, poorer adhesion of the fiber matrix to the liner and, on the other hand, problems with strength.
It is accordingly the object of the present invention to overcome the disadvantages stated above and to provide a pressure vessel which has a jacket consisting of fiber material with a high fiber volume density, optimize adhesion of the fiber matrix and improve stiffness, thereby enabling the wall thickness and hence overall weight of the pressure vessel to be reduced.
This and other objects are achieved by a method for producing a pressure vessel from a metal liner, which is reinforced at an outer lateral surface thereof by fiber composite material having a resin matrix, wherein, in at least one production step, the resin matrix of the fiber composite material is subjected to an ultrasound treatment.
The basic concept of the present invention is to increase the degassing rate of the air or air bubbles enclosed in the resin system during the production process, preferably during the process of winding the fiber composite material onto the outer jacket of the liner. This is done by an ultrasound treatment in order thereby to reduce the quantity of residual gas in the resin system. This leads to lower porosity of the fiber composite material and to increased stiffness.
Thus, according to the invention, a method is proposed for producing a pressure vessel consisting of a metal liner, which is reinforced at an outer lateral surface thereof by a fiber composite material having a resin matrix, preferably a CFRP fiber composite material, wherein, in at least one production step, the resin matrix of the fiber composite material is subjected to an ultrasound treatment.
In an advantageous embodiment of the invention, the characteristic of the ultrasound waves during the ultrasound treatment is chosen so that a maximized degassing rate of the enclosed air or air bubbles from the resin matrix is achieved. For this purpose, the intensity and wavelength of the ultrasound waves can be matched to the geometry of the liner and the properties of the material. As compared with a method that does not employ ultrasound treatment, the degassing rate and residual gas density can be determined by suitable analytical methods. A similarly suitable measure of demonstrating successful degassing is porosity determination, which can involve destructive or nondestructive measurement. By way of the ultrasound treatment measures according to the invention, porosity can be reduced to a value of about 0.05 to 0.75 (about 5% to 75%) compared with an untreated resin matrix.
In a preferred embodiment of the method according to the invention, the fiber composite material is applied to the outer lateral surface of the liner by a winding method, preferably a wet-winding method.
It is advantageous if the ultrasound treatment is carried out during the winding process. As a supplementary measure, the liquid resin bath or the impregnated resin matrix can be treated with ultrasound waves before the winding process.
In another advantageous embodiment of the method, an ultrasound treatment of the resin of the resin matrix can take place (continue) even during the curing process of the resin, preferably in a vacuum chamber. Through the use of a vacuum during winding and/or during curing, degassing is further promoted, allowing low residual gas densities to be achieved.
During the ultrasound treatment, the ultrasound waves are advantageously input during the ultrasound treatment by at least one ultrasound probe coupled mechanically or acoustically to the metallic liner. It is also possible, for example, to use two ultrasound probes, wherein the ultrasound probes are coupled respectively to an inlet stub and an outlet stub of the liner of the pressure vessel.
In a preferred embodiment of the method, the liner is rotated about its axis, preferably about the center line of a rotationally symmetrical pressure vessel, to enable the liner to be wound with the fiber composite material. It is particularly advantageous here if the ultrasound waves are input in such a way that the increase in the degassing rate due to ultrasonic excitation takes place only in that region of the upper half shell which is at the top during the rotation of the liner. This avoids a situation where gas which emerges on the underside cannot escape upward, which would counteract the desired effect. This can be accomplished through a suitable choice of ultrasound sensors and the appropriate coupling thereof to the liner so that only the upper half shell is subject to the desired ultrasound resonance.
A particularly advantageous embodiment consists in the combination of an aluminum liner with fibers of CFRP. However, another advantageous possibility in the case of different metallic materials of the liner is for the fiber composite material to consist of carbon fibers and/or glass fibers.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.